The increasing development of digital video technology presents an ever increasing problem of reducing the high cost of video compression codecs (coder/decoder) and resolving the inter-operability of equipment of different manufacturers. To achieve these goals, the Moving Picture Experts Group (MPEG) created international standards 11172 and 13818, which are incorporated herein in their entirety by reference.
In the area of rate control, MPEG does not define a specific algorithm for controlling the bit rate of an encoder. It is the task of the encoder designer to devise a rate control process for controlling the bit rate such that the decoder input buffer neither overflows nor underflows. Thus, it is the task of the encoder to monitor the number of bits generated by the encoder, thereby preventing the overflow and underflow conditions.
Currently, one way of controlling the bit rate is to alter the quantization process, which will affect the distortion of the input video image. By altering the quantizer scale, the bit rate can be changed and controlled. Although changing the quantizer scale is an effective method of implementing the rate control of an encoder, it has been shown that a poor rate control process will actually degrade the visual quality of the video image.
In the current MPEG coding strategies, the quantizer scale for each macroblock is selected by assuming that all the pictures of the same type have identical complexity within a group of pictures. However, the quantizer scale selected by this criterion may not achieve optimal coding performance, since the complexity of each picture will vary with time.
Furthermore, encoders that utilize global-type transforms have similar problems. For example, one such global-type compression technique appears in the Proceedings of the International Conference on Acoustics, Speech and Signal Processing, San Francisco, Calif. March 1992, volume IV, pages 657-660, where there is disclosed a signal compression system which applies a hierarchical subband decomposition, or wavelet transform, followed by the hierarchical successive approximation entropy-coded quantizer incorporating zerotrees. The representation of signal data using a multiresolution hierarchical subband representation was disclosed by Burt et al. in IEEE Trans. on Commun., Vol Com-31, No. 4, April 1983, page 533. A wavelet pyramid, also known as critically sampled quadrature-mirror filter (QMF) subband representation, is a specific type of multiresolution hierarchical subband representation of an image. A wavelet pyramid was disclosed by Pentland et al. in Proc. Data Compression Conference Apr. 8-11, 1991, Snowbird, Utah. A QMF subband pyramid has been described in “Subband Image Coding”, J. W. Woods ed., Kluwer Academic Publishers, 1991 and 1. Daubechies, Ten Lectures on Wavelets, Society for Industrial and Applied Mathematics (SIAM): Philadelphia, Pa., 1992. Furthermore, U.S. Pat. No. 5,412,741 issued May 2, 1995 and herein incorporated by reference discloses an apparatus and method for encoding information with a high degree of compression.
The output bit stream from a video encoder tends to have a variable bit rate that fluctuates according to scene contents and the nature of the coding process used by the encoder. As such, the encoder requires a mechanism to regulate the output bit rate to match the channel rate with minimum loss of signal quality.
Therefore, a need exists in the art for an apparatus and method to maintain the overall quality of the video image while optimizing the coding rate. Similarly, encoders that utilize global-type transforms such as wavelet transforms have special requirements that are not met by the prior art rate control techniques.